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Why do clouds Have Different Shapes?

Clouds vary(变化) in shape according to their height and temperature, and they contain minute(微小的) drops of water or ice particles(微粒) or a combination(混合体) of both. And, of course, their formation(构成) is greatly affected by wind changes.

There are basically three groups of clouds: high clouds between 17000 and 45000 feet; middle clouds between 7000 and 23000 feet; and low clouds up to 7000 feet. Their height and temperature decide how much pressure is exerted(施压) on them by the atmosphere.

Finally, the shapes of clouds differ according to the time of day.Towards evening clouds tend to thin out (变淡薄) , rise a little and flatten out(变平).

When is the Earth Nearest the Sun?

The earth is nearest to the sun on about the second or third day in January. The distance varies because the earth spins(旋转) round the sun in an elliptical orbit(椭圆形轨道) or path. The time when the earth is closest to the sun is called the perihelion(近日点). The time when it is farthest away ― the aphelion(远日点) ― comes six months later on the first or the second day in July. During the perihelion the earth is 147 million kilometres from the sun, but during the aphelion it is 152 million kilometres away.

When planetary (行星的;地球的) distances in the solar system(太阳系) are compared , the average distance between these two extremes(极)is used. In the case of the earth and sun, this is called the astronomical unit(天文单位) and measures 149 million kilometres.

If it was possible for an aircraft to fly from the earth to the sun at a constant speed of 1000 miles an hour, it would have to travel for over10 years to reach its destination(目的地).

The earth takes 365.25 days to travel around the sun and moves at a speed of nearly 19 miles a second. As can be seen from the dates of perihelion and aphelion, the nearness of the sun does not determine the seasons. Our seasons are decided by the amount of daylight and directness of the sun’s rays on the earth’s surface.

These conditions vary because the axis(轴) on which the earth spins is tilted(倾斜).

What is Twilight?

Twilight(曙暮光) is the faint light which appears a little before sunrise and again after sunset before it gets really dark.

On the moon there is no twilight. Darkness comes suddenly as soon as direct sunlight ceases(停息) to reach the moon’s surface. This does not happen on earth because of the halo(光辉) of air called the atmosphere(大气层), which surrounds it. When the sun goes below the horizon(地平线), its light leaves the earth but it reflected(反射) downward from the upper atmosphere.

Poets and writers have written of the evening twilight ― or “the gloaming” as it is called in Scotland as an enchanted(令人陶醉的)time. Perhaps one reason is that familiar objects become distorted(歪曲的,变形的)in the half-light(暗淡的光线) and we imagine we are seeing things that are not really there.